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Radio Scripts

Radio Scripts

Package 89, Script 2
December 2009

Soil fertility and climate change: An issue pack

Notes to Broadcaster

Soil fertility is declining in many parts of sub-Saharan Africa. Climate change is expected to bring more extreme weather events such as flooding, drought, and more unpredictable weather. These changes will likely only deepen problems with soil fertility.

Soil fertility is declining for a variety of reasons. Some common farming practices, including burning crop residues and leaving soil bare and unprotected from the sun and wind, are part of the problem. Excessive or insufficient use of fertilizers and improper crop rotations also lead to declining soil fertility.

But there are many traditional and modern practices which can help boost soil fertility, and assist farmers in making their farms more resilient and resistant to the changing climate. These include micro-dosing of fertilizer, using rather than burning crop residues and other organic matter, planting nitrogen-fixing crops and trees, making good use of compost and manure, and taking steps to prevent wind and water erosion.

A good soil fertility strategy for farmers is to make best use of all organic sources of plant nutrients. It is also wise to use practices which conserve soil and water, and, when possible, to make careful use of small amounts of expensive inorganic fertilizers. Organic sources of nutrients include legumes, green manures and cover crops, composts, and animal manures. The best practices will vary by region, and will often build on local knowledge.

This issue pack begins with two true stories about farmers and soil fertility. It then offers some background information on soil fertility. Next, it suggests some starting points for creating locally relevant radio programs on the subject. Finally, it points to various resources on soil fertility – radio programs, documents, and organizations. 

Samison Banda, from Mchinji, Malawi, manages his farm with his sister and two nephews. Because his fields lie on a hillside, his ridges and crops were often washed away by heavy rains in the past. "I used to plant three to four times every year because ridges were broken and crops were washed away by rainwater. The size of my land decreased every year. My field was cut by gullies which grew deeper and wider every year." Banda’s yields declined and his family fell deeper into food insecurity.

In 1993, he took part in a project which offered training in better farming and agroforestry practices. He began to build ridges on hill contours, made tied ridges, planted erosion-controlling vetiver grass, sowed leguminous Tephrosia shrubs, and used improved fallows. In 1994, he planted Faidherbia albida trees on half his land to improve fertility. After several years, soil loss had decreased tremendously, while soil fertility and soil structure had strongly improved. After eight years, the Faidherbia trees were large enough to contribute to soil fertility and provide domestic firewood.

"Now I harvest enough with little or no chemical fertilizers," says Banda. "My garden preserves enough moisture throughout the year, even when there are dry spells. I am usually the last person to suffer from drought in this catchment," he boasts. Tephrosia also gives him fuel wood, pesticides from the leaves, and cash from selling extra seed. With this money, he has bought clothes for his family, a new radio, and two bags of chemical fertilizer.

Mama Susanna Sylvester lives in the Kondoa district of Dodoma region, in central Tanzania. Annual rainfall is very low in the area. This elderly woman heads a household that farms six hectares of land. Susanna used to be relatively poor, like most of her neighbours. But this creative farmer developed a composting system which has served her well.
She mixes crop wastes with fodder residues, and adds manure and urine from her zero-grazed cow and stall-fed pig. She puts the mixture in large pits, and then adds ash and wastewater from household washing to keep the compost moist. Her compost has made her land much more fertile. She has even been able to sell surplus compost. She states that over 30 farmers, both men and women, have followed her novel technique for making compost.

 

Fertile soil has the following properties:

How is soil degraded?
When the nutrients which lead to soil fertility are removed and not replaced, or when the conditions that support soil fertility are not maintained, soil becomes degraded. This leads to poor yields.
African soils are often low in nutrients to begin with. If you add to that the combined effects of shorter fallow times and continuous cropping, slash-and-burn agriculture, and other factors, soils can become seriously degraded or depleted.
Soil degradation can occur through too much tilling of the soil, which damages soil structure. Also, overusing inputs such as synthetic fertilizers and herbicides can leave residues and buildups that hinder the work of microorganisms. A buildup of salt in the soil, often associated with irrigation, can deplete fertility and limit crop yields.
Soil which is left bare after burning residues or harvesting crops is vulnerable to erosion from wind and rain. The topsoil that is washed or blown away contains most of the soil nutrients.

In dry areas, some soils are severely degraded. Some bare and crusted soils are virtually “dead.” Yet a project in Burkina Faso showed that farmers can achieve good success even with these kinds of soils. The farmers applied mulch to attract termites. The termites then broke up the hardened soil, which increased water infiltration. The land became productive enough to farm within months.

A little soil science

The physical characteristics of the soil determine how much water a soil will hold. Soils which hold a good amount of water have many small pores which hold water until it can be taken up by plant roots. Soil must also be adequately aerated. Air is stored in larger pores in the soil. Soils which have been compressed by the constant weight of equipment or livestock will not have a good structure, contain little water or air, and therefore have poor fertility.

Soil chemistry affects the availability of plant nutrients in the soil. The pH level of the soil, together with its aeration, affects the form in which nutrients are found in the soil. A measure called CEC (cation exchange capacity) indicates the amount and type of clay in soils, as well as organic matter content. A good CEC level indicates that a soil can hold nutrients in a form which is readily available for uptake by plant roots. In general, soils which contain higher amounts of clay (rather than sand), and a high percentage of organic matter, have better CEC levels.

The biological life in the soil – soil-dwelling macro-organisms and micro-organisms – breaks down crop residues into organic matter. A healthy biological life in the soil also limits many plant diseases and soil-dwelling pests which damage crops.

The level of organic matter in the soil is critical to soil fertility. Organic matter levels affect soil structure, cation exchange capacity, the amount of water that the soil can hold, and the level of nutrients available for plant growth.

Managing soil fertility: principles and practices

A system called integrated plant nutrient management recommends that farmers use the following three principles for best soil fertility:

Using organic matter to fertilize the soil: How it works

Micro-organisms decompose organic materials such as crop residues. This decomposition releases nutrients for uptake by plant roots. Nitrogen is mineralized by micro-organisms. This means that micro-organisms decompose the nitrogen, turning it into forms which are easier for plants to use. But micro-organisms themselves consume nutrients such as carbon and nitrogen.

All organic materials are made up of about half carbon. But the level of nitrogen varies widely between different types of material. In general, organic material that is old and tough has a high carbon to nitrogen ratio (C:N ratio). So the nitrogen content is low compared to the amount of carbon. Young and succulent material generally has a low C: N ratio, and thus a higher nitrogen content.

If old and tough organic material (such as straw) is added to the soil or to a compost pile, the soil micro-organisms will first need more nitrogen than can be released from the straw. So they will absorb not only all of the nitrogen that is released from the straw, but also all of the nitrogen that is available in the soil. Thus, after straw is incorporated into the soil, there is a period of time in which all of the available nitrogen in the soil is taken up by the micro-organisms. This is called immobilization. Little or no nitrogen is then available for the plants. When the straw is completely decomposed, there is no longer enough food available for all of the micro-organisms. A large proportion of the micro-organisms then die and are themselves decomposed. The nitrogen that the micro-organisms had immobilized is then available for plants. In warm, moist conditions this cycle occurs quickly, and the period of immobilization is short, measured in weeks. In drier areas, the period of immobilization is longer, requiring more than a growing season.

Legume crops

Legumes add nitrogen to the soil, mostly through falling leaves, but also when their roots and nitrogen-rich root nodules decompose underground. The organic matter produced by legumes is rich in nitrogen, decomposes quickly and is a good source of nitrogen for other plants. Herbaceous (non-woody) green manure legumes (often called cover crops) and fast-growing leguminous trees are excellent ways to improve soil fertility. Cover crops also provide a dense soil cover that can prevent soil erosion, evaporation of water, and suppress weeds.

But legumes need good conditions to grow and farmers may have to improve the soil first, so that legumes can contribute to soil fertility. The most common problem is a shortage of phosphorus. In highly acid soils, liming or adding animal manure can raise the pH and increase the availability of phosphorus. In most soils, however, the only option is to add phosphorus. As phosphorus fertilizers are not normally affordable for African farmers, a good strategy is to use rock phosphates. A farmer should be careful to obtain sources of rock phosphate that are effective at providing nutrients for his or her crops. Sources of rock phosphate may be obtained through extension officers or district agricultural offices. 
Some soil fertility practices are indigenous to particular regions. For example, in the Usambara Mountains in north-western Tanzania, farmers use a local plant called Tighutu to increase soil fertility (see resource 17). In central India, farmers spread groundnut shells on cattle bedding and remove them when they are soaked with urine, then mix them with dung (see resource 18). In areas near Lake Victoria and elsewhere, water hyacinth is used as a base for making compost (see resource 19).

 

Farmer science – tests for determining the best way to use different kinds of organic materials to fertilize the soil

Scientists analyze the quality of crop residues in a laboratory. They use equipment to conduct tests which measure the amount of nitrogen, carbon, lignin and other substances in the residues.

But tests can also be conducted by farmers. For example, nitrogen levels in leaves and other material can be estimated simply on the basis of colour. Dark green leaves are higher in nitrogen, and make good fertilizer. Yellow leaves are lower in nitrogen and do not, by themselves, make good fertilizer.

Using this simple nitrogen test and simple tests for two substances – lignin and polyphenols – results in the following guidelines for use of organic materials:

 

Using these kinds of tests and guidelines at a farmer field school in Kenya, farmers picked fresh green leaves of hedgerow plants and incorporated them into compost heaps to speed up decay. Nitrogen-rich organic residues were in very short supply for these farmers. These materials were mostly used to fertilize high-value tomatoes and cabbages. Maize stover was used as fuel for cooking, particularly by poorer households. Farmers with lots of poor-quality organic materials used it as bedding for cattle, and eventually added it to the manure heap, or used it as mulch to assist in soil conservation. The farmers learned that more succulent and fibrous leaves or material such as sisal and Euphorbia decayed slowly, were hard to crush, difficult to compost and of little use for nutrient management.

  

Two more success stories

The Gacheru Self-Help Group, a farmers’ group in central Kenya, has benefited by using a fertilizer made from local weeds. According to the farmers, the organic fertilizer solution works well and is far cheaper than inorganic fertilizers. The process for making the solution was developed by the Kenya Agricultural Research Institute and involves extracting juice from two local weeds, Desmodium and Tithonia. These weeds are high in nitrogen, which boosts crop yields, says farm manager Samuel Mugo. "They fight all manner of destructive weeds and give us high yields from our farms." The process is simple. The farmers thresh the weeds, then mix them with water in a drum. The solution is then ready to use.

Zambian farmers use termite soil to improve soil fertility: Zambian farmers traditionally used soil from termite mounds as a fertilizer. But when inorganic fertilizers became available at subsidized prices, this practice declined in popularity. When the subsidies were removed, many farmers resumed the practice.

A suitable termite mound is chosen and cleared of vegetation. Soil from the top of the mound is removed, but the base is left intact so that the colony is not destroyed. The soil is transported to the field and, before the rains begin, farmers work it into the topsoil with ploughs, hoes or shovels. Where farmers practice conservation farming, the soil is placed in planting pits.

The termite soil is applied once every three years. Maize harvests were 33 percent higher in fields with termite soil than in fields where inorganic fertilizers were used. Termite mound soils have lots of clay, which improves water storage. In southern Zambia, soils are normally poor at holding water, so using termite soil helps hold more water and improve crop yields. Termite soils also have high levels of calcium, phosphorus and organic matter, which all contribute to better crop yields and health.

Other useful practices for improving soil fertility include:

 

Micro-dosing of inorganic fertilizer

Inorganic fertilizer is often too expensive for small-scale farmers, especially the recommended dosages often suggested by extension agents or fertilizer suppliers. However, recent research has shown that, when carefully applied, much smaller doses of inorganic fertilizer can have a strongly positive effect on crop yields. This is called micro-dosing. Together with extensive use of organic materials and efforts to conserve soil and water, inorganic fertilizer can be a useful input.

Micro-dosing is simple. Here’s how it works: Fertilizers such as di-ammonium phosphate (DAP) or NPK (nitrogen-phosphorus-potassium fertilizers) are placed in planting holes along with the seed, and then covered with soil. In West Africa, farmers found a labour-saving method for micro-dosing. While one farmer walks through the field making planting holes, a second follows him or her with two vessels: one with the seed and the other with DAP or NPK. He plants the seed, adds a three-finger pinch of fertilizer, and pushes the soil over the hole with his feet. The three-finger pinch of fertilizer is about two grams, although it should be added that in some areas smaller amounts have significantly increased yields. The micro-dosing is supplemented with one gram of urea per plant three weeks after sowing. In the West African Sahel, micro-dosing has resulted in yield increases of between 44 and 120% for pearl millet and sorghum.

 

There are many ways to create radio programming on soil fertility. Here are a few:

 

 

 

 

Radio programs

 

Documents

 

Organizations

Talk to people with expertise in soil fertility at many organizations, including:

Acknowledgements

Information sources

Canadian Flag Program undertaken with the financial support of the Government of Canada provided through the Canadian International Development Agency (CIDA)